High-efficiency metal filter element and process for the manufacture thereof

Abstract: The present invention is a high-porosity metallic membrane element, comprising a sintered element having at least about 60% porosity, the sintered element comprising a matrix of substantially interconnected pores, each of the pores being defined by a plurality of dendritic metallic particles. A preferred form of the invention is made from pure nickel, preferably filamentous nickel powder. An article of manufacture made from the element includes a high-porosity metallic membrane filter, comprising the aforementioned sintered element having at least about 60% porosity, a filter housing, the housing thereby defining a filtered fluid flow path; and means for sealably connecting the casing to a fluid to be filtered. Another aspect of the invention is a method of making the high-porosity metallic membrane element, comprising the steps of depositing a substantially uniform low-density bed of a sinterable dendritic material into a mold suitable for applying compressive force thereto; compressing the low-density bed of sinterable dendritic material to form a green form; and sintering the green form. Filters made in accordance with this invention exhibit superior porosity, outgassing, and particle shedding characteristics when compared to the state-of-the-art.

Description: Background of the Invention 1. Field of the Invention The present invention generally relates to a fluid filter and a process for making it. In particular, the invention is a novel all-metal gas filter with high efficiency and low outgassing characteristics that is useful as a point-of-use filter for semiconductor process gases. 2. Description of the Prior Art Semiconductor manufacturing is constrained by the limitations of purity. In chemical vapor deposition of the alternating layers of silicon and dopant, a critical aspect of the process involves the absence of any particulate impurities. The presence of a minute particle can destroy an entire silicon wafer representing many dollars of potential end-product. To that end, an entire industry has developed concerned with only one thing-the filtering of the gases that may come into contact with semiconductor product during its formation. Clean rooms equipped with HEPA (High Efficiency Particulate Air) filters are the first line of defense. Process equipment is located within "clean rooms" that are filled with carefully filtered air. The design of the equipment itself endeavors to minimize particle shedding, outgassing, and contamination from the materials used to transport and deliver high-purity gases such as silane, arsine, hydrochloric acid and phosphine. An important component in these delivery systems is the filter which insures that particulate contamination does not reach the point where the gas is discharged onto the work (point of use). These filters must not only remove any particulate material, but also must not add any gaseous contamination to the high purity gases. In addition, the gas delivery systems must also be as compact as possible to eliminate contamination, both particulate and gaseous, which might arise from either the installation of such systems, or the normal wear associated with usage. This is especially the case with highly corrosive gases such as hydrochloric acid. Therefore the filters must not only remove particulate material and not be a contributor of gaseous impurities, but they must also be as compact as possible and have small internal and filter volumes. Various filters are used for filtration of such gaseous fluids to insure ultra high levels of purity in terms of particulate contamination. These include: organic membrane filters, ceramic filters, filters formed from porous metal structures and filters formed from metal fibers. Although some of these various filter medias are capable of providing particulate contamination control to levels less than one part per million or greater in terms of particulate control, due to the large flow area required to sustain flow at reasonable pressures and maintain low face velocities to insure particulate retention, gaseous impurities such as moisture, oxygen and especially hydrocarbons are often present in detectable levels (parts per million). This contamination can occur during manufacture of the filter, during installation of the filter when it is exposed to an atmosphere other than a high purity gas, or even as a result of outgassing from the material the filter is packaged in. In addition, such large filter volumes require relatively larger housings to contain them. This in turn results in a greater likelihood of contamination due both to installation and usage and the need for larger gas delivery systems to fit the filters. Present metal filters include stainless steel, nickel, or nickel alloy sintered-powder types such as the Waferguard II SF™ (trade of Millipore Corporation, Bedford, MA), and the Mott GasShield™ (trademark of Mott Mettalurgical Corporation, Farmington, CT)( See U.S. Patent No. 5114447 to Davis) line of filters. Such filters, being all metal, exhibit low outgassing, high efficiency, corrosion and temperature resistance, medium porosity and gas throughput, and high structural strength. However, low porosity has continued to be a drawback for typical sintered metal powder filter elements. Porosities for the above filters range from 40 to 44%, limiting the flow-through characteristics of these filters. The low porosities are inherent in the processes used to manufacture sintered metal powder filters. Typically, the powders are compacted into a mold to form a "green form," then sintered to join the metal particles together to impart the necessary strength. The final filter elements (or "membranes") may be cut from a flat sintered sheet of metallic powder, or molded into the final shape in the molding step. The temperatures at which the sintering proceeds are a critical factor in determining the final porosity. Higher temperatures lead to increased strength, but lower porosity; lower temperatures lead to decreased strength and higher porosity. Until now, the final porosity was limited to about 45% in the sintered metal powder art. There exists a need for increased porosity in the metal filter art. Increased porosity would allow for the construction of smaller filters with all the positive aspects of highly porous metal filters, with less outgassing and particulate shedding problems. Reference is also made to DE-A-4 102 430 which teaches metal filters with porosities greater than 45%. The manufacture of these filters requires the use of additives. Summary of the Invention In accordance with the present invention it has been found that highly porous filters made from filamentary metal powders can be made which require a fraction of the filter volume found in existing filters. The filters of the present invention retain the high level of particulate efficiency (less that one part per million passage through the filter) found in existing filters, while operating at substantially higher face velocities. In addition, the gas pressure required to sustain these velocities is equivalent to or less than existing filters. The filter of the present invention has minimal internal volume and is highly compact, resulting in very low contaminant generation due to exposure to atmospheres other than high purity gases and the wear associated with usage. In addition, the present invention shares other advantageous characteristics of porous metal filters, namely good mechanical and thermal properties allowing operation at elevated temperatures and high differential pressures, and the absence of particulate shedding associated with prolonged usage. The present invention is a high-porosity metallic membrane element according to claim 1, comprising a sintered element having at least about 60% porosity, the sintered element comprising a matrix of interconnected pores, each of the pores being defined by a plurality of dendritic metallic particles. A preferred form of the invention is made from dendritic nickel powder and has a porosity in excess of 65%. In a preferred embodiment a low sintering temperature of from about 675° to about 725°C is employed. No binder is employed, practically eliminating outgassing in the final membrane element. The invention also includes a high-porosity metallic membrane filter. The filter includes a sintered element having at least about 60% porosity, the sintered element comprising a matrix of interconnected pores, each of the pores being defined by a plurality of dendritic metallic particles. The filter also includes a filter housing defining a fluid conduit, the conduit being a casing for retaining the filter element in the fluid flow path, the casing having a front and a back with the element being located between them, the filter element being welded to the casing wall. Means for sealably connecting the casing to a fluid to be filtered are provided by utilizing typical Swagelock∨-style connectors. Another aspect of the invention is a method of making a high-porosity metallic membrane element according to claim 2, including, but not limited to, the steps of first depositing a substantially uniform low-density bed of a sinterable dendritic material into a mold suitable for applying compressive force thereto. The second step is compressing the low-density bed of sinterable dendritic material to form a green form. The last step is sintering the green form at a temperature below the melting point of the metallic material. In a preferred method of the invention, depositing the substantially uniform low-density bed includes, but is not limited to, the steps of first air-laying the sinterable dendritic material into a mold suitable for applying compressive force thereto, the air-laid bed having a density less than or equal to the density of the sinterable dendritic material. In a preferred embodiment the sinterable dendritic material is filamentary nickel powder. Compressing the bed of nickel is accomplished at a pressure below about 3448 kPa (500 psi), in the absence of any binder. In another preferred embodiment, the sintered green form is compressed at a second higher pressure, thereby imparting additional structural rigidity to the element. The higher pressure is generally in the range of 4138-7586 kPa (600-1100 psi). The final porosity of the sintered membrane element is generally from about 60% to about 70%. Accordingly, it is an object of the present invention to provide a highly efficient high-porosity metallic membrane element useful as a point-of-use filter in the semiconductor industry. The high efficiency of the invention allows a substantial savings in space utilized in the process instrumentation, reduced outgassing due to decreased internal surface area, and reduced pressure drop over the length of the filter. It is another object of the invention to provide a method for forming high-porosity metallic membranes with excellent structural strength, very high porosity, and without the use of any binders. It is still another object of the invention to provide a membrane element that may be formed in any of numerous shapes for inclusion into a filter housing. Brief Description of the Drawings Figure 1(a) is an electromicrograph of the surface of a sintered nickel membrane element made in accordance with the present invention. A 10 scale is provided for purposes of illuminating the relative sizes of the pores. Figure 1(b) is an electromicrograph of the surface of a competitor's sintered nickel membrane element, taken under the same conditions as Fig. 1(a). Pore sizes appear similar, but the apparent pore density is visibly lower. Figures 2 (a-f) are schematic representations of the process for producing a metallic membrane element. Fig. 2(a) depicts the air-laying step and apparatus used therein. Fig. 2(b) depicts the second step of a low-pressure compression of the air-laid powder into a green form. Porosity (PO) of the green form is about 80-90%. Fig. 2(c) depicts the high-temperature vacuum bake/hydrogen reduction, resulting in a sintered green form with a PO ranging from 70-80%. Fig. 2(d) shows the second compression at a higher pressure, typically from 4138-7586 kPa (600-1100 psi). PO decreases to between 60-70%. Fig. 2(e) shows the cutting step. Wire EDM cutting is depicted. Fig. 2(f) shows the final low-temperature vacuum bake step. Figure 3 is a graph of total hydrocarbons (ppb) versus time (minutes) representing total hydrocarbon outgassing from the Mott Grasshield™ model POU-05 and Millipore WG III-NF metal filters. Figure 4 is a graph of face velocity (slpm /cm 2 ) versus the log reduction values (LRV) for the invention, and for various competitor's products. The invention (labeled "nf") shows higher LRVs at all face velocities. Competing products are the "sf" filter (Wafergard II SF Mini Inline Gas Filter, Millipore Corporation, Bedford, MA); Ultramet-L Gaskleen:, model # GLFF4000VMM4, (Pall Corporation, East Hills, New York); Mott GasShield model POU-05 (trademark of Mott Mettalurgical Corporation, Farmington, CT). Figure 5 is a graph plotting face velocities (slpm/cm 2 ) against pressure for the same products as in Fig. 4. Figure 6 is a cross-sectional schematic of the filter device prior to welding the membrane element to the inner circumference of the filter housing. Figure 7 is a cross-sectional schematic of the filter device after welding the membrane element to the inner circumference of the filter housing. Note that the porosity of the element is largely intact. Detailed Description of the Preferred Embodiments The invention provides a high efficiency high-porosity all metal filter membrane and a method for making it. Filters incorporating the membrane exhibit increased throughout, decreased pressure loss, substantially lower outgassing, and decreased size and complexity. These characteristics make this filter ideal as a point-of-use process gas filter in the semiconductor, and related, industries. The following terms are used in this application. The term "porosity" is defined as the amount of pore volume per total volume of the membrane element. Per cent porosity is the pore volume divided by the total membrane element volume, multiplied by 100. As used herein, the term "metallic" is meant to describe all metal-containing materials. This includes , but is not limited to, pure metals, mettaloids, metal oxides, metal alloys, and similar combinations that would be obvious to one of ordinary skill in the metalworking art. As used herein the term "membrane element" describes the wafer-like product of this invention. It is characterized in that it has a high internal pore volume, excellent structural rigidity and strength, and low pressure drop when fluids, specifically gases, are passed through it. As used herein the term "sintered" refers to the process for bonding compacted metal particles together by raising their temperature to a point close to but less than their common melting point. Although applicants are not bound by any underlying theory of the present invention it is believed that the driving force for sintering is the decreased total surface area of the individual particles as compared to the sintered form. In general, the term "sintered" describes the process of heating metallic particles or powders that are loosely organized in an array to a point below their melting point in an inert or reducing atmosphere. This process has in the past required a binder to maintain the metallic particles in proximity to one another. As used herein the term "matrix" describes a physical structure comprising a network of interrelated pores or crevice-like areas. The matrix, while generally a uniform structure, is not necessarily 100% uniform. Some pores may be completely unconnected to other pores, and thus may not be connected to the matrix. "Substantially interconnected" can this be interpreted to mean that the majority of the pores will touch upon or share elements of at least one other pore, thereby allowing communication between the pores. As used herein the term "dendritic" refers to the tree-like appendages that the metallic particles display. The term comes from "dendrite" which is a highly branched nerve cell. The dendritic property of the metallic powder makes it possible to attain a higher pore area than with non-dendritic metal powders because of the interactions of the branches with each other. Other metal powders which are or may be made dendritic come within the scope of this invention. As used herein the term "green form" is a term well-known in the sintering art. It refers to the compressed metal powder structure before it is sintered. The green form displays a higher porosity than the final sintered membrane, but is very fragile. As used herein the term "sinterable dendritic material" refers to any matter whose individual particles have a highly branched external surface, the particle also being capable of being sintered. As used herein the term "substantially uniform" when used to describe the low-density bed means that there are few or no significant local variations in the density of the airlaid bed. A significant variation is one which would cause... With reference now to Figure 1(a), an electromicrograph of a sintered metallic membrane element made in accordance with the method of the invention is shown. The membrane depicted is comprised of a filamentary nickel powder. In a preferred embodiment a filamentary nickel powder such as commercially available INCO 255 (Novamet Speciality Products, Wyckoff, NJ) is used. While the present invention is primarily described as employing filamentary nickel powders with a Fisher size of between 2 and 3µm (ASTM B 330), it is understood that any metal powder having the physical characteristics of this filamentary, dendritic powder can be employed to produce highly porous metal membranes using the same techniques described herein. Other filamentary metal powders may also be used or later developed which may be successfully used in this invention. The size of the pores shown in Figure 1 ranges from about 2 to about 10 µm. The density of the starting filamentary powdered metal is typically 0.5 to 1.0 g/cm 3 . The final product density is between about 2.75 and 3.0 g/cm 3 . Figure 1(b) is a similar electromicrograph of a competing nickel filter taken under similar conditions. Note that the pore size is approximately the same as the present invention, but the pore frequency is visibly less. This is reflected in the fact that the density of the competing product filter is 5.0 - 5.75 g/cm 3 , while the density of the membrane of the present invention is from about 2.75 to 3.0 g/cm 3 . Figure 2 is a schematic generally describing the process of making a metallic membrane element. In accordance with the process of the present invention, a highly porous metal membrane is prepared by employing a variety of novel operations. Generally, the steps of the process include depositing a uniform bed of metal powder by air-laying a bed of sinterable dendritic metallic powder to form a highly uniform, exceeding low-density powder bed whose apparent density is equal to or less than that of the powder as it is obtained from its container. The next step is the low-pressure press of such a bed to form a high-porosity semi-self supporting green form, and then the low temperature sintering of the green form to produce a self-supporting highly porous metal membrane with porosity exceeding 60%. Fig. 2(a) depicts the first step of air-laying. As mentioned above, the formation of an exceedingly low-density bed of high uniformity is accomplished through the use of air-laying procedures. The term "air-laying" as used herein refers to the procedure whereby a pre-determined mass of powder is sifted through a screen sieve and allowed to fall under the influence of gravity into a mold below of fixed volume. Since the powder is "fluffed up" in this manner, its density becomes lower than that of the packaged powder. The distance the powder falls before coming into contact with the mold will necessarily vary depending on the area and shape of the mold. A variety of individual molds can be used depending upon the final shape and size of the desired product. For example, a 13 cm diameter round mold requires a falling height of at least 25 cm when using filamentary nickel powders to insure a bed of substantially uniform thickness and density. The term "substantially uniform" a larger diameter mold would require a larger falling distance. One of ordinary skill in the art will be able to determine this height through the use of routine experimentation, given the examples set forth herein. The bed, formed in the manner described, has a density equal to or less than that of the apparent density of the powder. Apparent density is determined following the procedure outlined in ASTM B 329. For filamentary nickel powders with an apparent density of approximately 1.0 g/cm 3 the air-laid bed density can be as low as 0.7 g/cm 3 . Now with reference to Fig. 2(b) the air-laid bed formed as described is then pressed to the desired thickness using relatively low pressure. The resulting porosities range from 80-90%. The degree of pressure required necessarily depends on three variables, namely the density of the air-laid bed, the thickness of the bed, and the desired thickness of the pressed green form. For example, for a bed with a density of 0.8 g/cm 3 and a thickness of 0.6 cm, a force of 2966 kPa [30 kg/cm 2 (430 psi)] is required to obtain a green form thickness of 0.4 cm. The density of such a green form would be 1.3 g/cm 3 and have a porosity of 85%. This green form would be self-supporting only to the extend that it can, with care, be removed from the mold while retaining its structure. However, the introduction of relatively minor stress on the green form will cause it to lose its integrity. Now with reference to Fig. 2(c), the self-supporting green form is given additional strength through the sintering step depicted therein. Generally, sintering is accomplished by heating a metallic powder to a temperature below its melting point in the presence of an inert or reducing atmosphere, or in vacuum. One of ordinary skill in the art of sintering will be able to determine the specific atmospheric conditions in which to sinter. The temperature and, to a lesser degree, the duration of the sintering process are two factors which determine the final dimensions, and hence porosity, of the metal membrane. The porosity of the sintered product typically decreases to 70-80 %. This occurs as a result of both sinter bonding of the powder particles and shrinkage of the membrane. A lower temperature and shorter sinter duration result in a membrane with smaller degrees of both sinter bonding and shrinkage. For example, a green form with a porosity of 80% sintered at 950°C for 5 minutes results in a membrane with a porosity of 58%. The same green form sintered at 800°C for 5 minutes results in a membrane with 72% porosity. It is apparent that a decrease in temperature results in an increase in resultant porosity. However, a lower limit to temperature does exist in so far as a given metal will not sinter unless given sufficient heat. In a preferred embodiment of the invention, the lower limit for sintering for filamentary nickel powder with a Fisher size of 2-3 µm has generally been found to be between about 500 and 600°C. A preferred form of the invention utilizes temperatures between about 675 and 725 °C, as this results in the appropriate membrane porosity and dimensions when combined with the proper formation of bed and green form. It is to be noted that one of ordinary skill in the art will recognize the interplay of these variables and will be able to adjust temperature and sinter time with no more than routine experimentation to find an optimum method, allowing the use of other sinterable dendritic materials. As shown in Fig. 2(d), the next step is to press the sintered green form to attain the final dimensions desired. The pressing is generally done at a higher pressure than the initial compaction step, given the fact that the sintered form is now much more rigid. The pressures generally used in this step are usually from about 4138-7586 kPa (600 to1100 psi). In a preferred embodiment, the sintered green form is pressed at greater than 6896 kPa (1000 psi). This step reduces the porosity of the product to its final porosity value. The final porosity is typically greater than 60%, and in a preferred embodiment greater than 65%. As shown in Fig. 2(e), the metallic membrane element may be cut into forms that allow construction of useful filters. In a preferred embodiment, wire EDM cutting is employed for this purpose. Wire EDM cutting is defined as the cutting of metals with a thin wire through which a high electrical current is passed. This method of cutting the final membrane element has given the best results. However, one of ordinary skill in the art will be able to adapt other cutting methods that may work equally well. For instance, cutting with an abrasive wheel or CO 2 laser may effect adequate separation. In a preferred embodiment the size of the nickel membrane element is approximately 1.2 cm in diameter and 0.25 cm in thickness. Currently, the size of the sintered sheet stock allows 16 elements to be cut from it. Referring now to Fig. 2(f), the membrane elements are low-temperature vacuum-baked to remove any volatiles introduced during the process. The temperature is usually less than 200°C. The final product exhibits minimal outgassing as depicted in Fig. 3. Fig. 3 is a graph of total hydrocarbons (ppb) versus time (minutes). When compared against a competitor's product (Mott POU-05 metal filter) outgassing from the product of the present invention is negligible. Outgassing was measured on a Beckman Model 400A THC Analyzer. The analyzer utilizes a flame ionization detector (FID) to determine the amount of total hydrocarbons (hydrocarbons of all molecular weights) present in a gaseous sample. In the present case, the gas was passed through the filter and on to the analyzer. The small diameter membrane element disks demonstrate high efficiency and flow rates. As depicted in Fig. 4, a filter made in accordance with the present method (designated "nf") clearly shows the high efficiency of a nickel membrane element. Fig. 4 is a graph of face velocity (standard liters per minute, hereinafter "slpm") versus log reduction value (LRV). LRV is defined as the log of the ratio of two numbers. In the present case, the ratio is that of the challenge of particles impacting the filter membrane element on the upstream side of the filter, to the number of particles detected downstream of the filter. Therefore, an LRV value of 7 would imply a number of 10 7 particles and the detection of 1 particle downstream, the log of this ratio being 7. The test is conducted by generating an aerosol containing several million particles with a size distribution centered around 0.014 m, passing this aerosol through the filter and counting those that pass with a condensation nucelus counter (CNC). The nickel membrane element of the present invention demonstrates excellent gas permeability. Fig. 5 is a graph of slpm/cm 2 versus pressure. Compared are the sintered metal powder filter of the present invention ("nf"), one from Mott Metalurgical Corporation (Mott GasShield), a sintered steel powder filter from Millipore Corporation, Bedford MA (model Wafergard II SF Mini Inline Gas Filter), and a steel mesh filter from Pall Corporation (Ultramet-L Gaskleen:, model # GLFF4000VMM4, East Hills, NY). Of the three sintered metal powder filters, the nf filter exhibits significantly greater permeability at all operating pressures above approximately 5 psi. At 10 psi, the nf filter has a face velocity of 2.4 slpm/cm 2 while the other two are at 0.8 and 1.5, respectively. At 138 kPa (20 psi) the difference is even greater, with the nf filter at 7.5 versus 1.8 slpm/cm 2 for the Mott filter. The filter device comprises a filter housing defining a fluid conduit, the filter housing comprising a casing for retaining the element in the fluid flow path, the casing having an anterior and a posterior with the element being located therebetween and being sealably joined to the casing wall, the housing thereby defining a filtered fluid flow path. Means for sealably connecting the casing to a fluid to be filtered are provided. These include Swagelok fittings and the like. The main requirement for the connecting means is that it be fluid-tight, i.e., does not leak. One of ordinary skill will be able to determine what means are appropriate under the particular connecting scenarios. Fabrication of filter devices incorporating the membrane elements is accomplished according to the following general procedure. First, a suitable material is selected for the filter housing. The material must be suitable for use under the ultra-clean conditions necessary in the semiconductor fabrication art. Stainless steel is a preferred material, although one of ordinary skill in the art is able to select a suitable filter housing material. The housing is usually constructed of two symmetric halves. The metal membrane element is situated between the two halves and welded in place using a process involving a "consumable lip." The lip is machined or cast into the outer surfaces of the two housing halves prior to assembly. The halves are then united with the membrane located opposite the consummable lip and touching the inner circumference of the housing as depicted in Fig. 6. The solid steel lip is then heated to melting, and then flows into voids within the filter membrane element. When it solidifies, the result is a solid steel-nickel weld bead which extends from the periphery of the filter housing to the interior of the membrane as shown in Fig. 7. Examination of photomicrographs of sectioned devices have shown this to be the case when correct welding parameters are followed. One of ordinary skill in the art is able to determine these parameters through routine trial and error. Having now generally described this invention, the same will become better understood by reference to certain specific examples which are included herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. All patents and publications cited herein are fully incorporated by reference in their entirety. Examples The following examples illustrates the effect of unsintered green form density and sintering temperature on the final porosity of the membrane EXAMPLE 1: Effect of varying the thickness of the air-laid bed on green form density This example shows that differences in the green form density are directly related to the amount of compaction selected during the initial compressing step. To start, three grams of filamentary nickel powder (INCO 255) were sifted into a 2.54 cm diameter mold as previously described under air-laying techniques, and two individual but identical green forms were made. The initial thickness of both air-laid beds was 0.3 - 0.4 cm. The first mold (green form #1) was pressed at 1035 kPa (150 psi) to a green form with a thickness of 0.373 cm and a porosity before sintering of 82%. The same mold with the same air-laid bed of INCO 255 nickel powder is compressed again at 3448 kPa (500 psi) to a thickness of 0.261 cm and a porosity of 74% before sintering. Both are sintered in vacuum at 675°C for 20 minutes. #1 has a final porosity of 75% (a change of -7%), #2 a final porosity of 63% (a change of -11%). Clearly there is a direct relationship between the pressure applied to the INCO 255 nickel powder and the final porosity. With this teaching one of ordinary skill in the art is able to determine with routine experimentation what pressure will be necessaary to attain a certain desired final sintered density. EXAMPLE 2 : Effect of varying the weight of the green form on the final density Adding weight (i.e. more powder) to the mold results in a thicker product with higher density. Two green forms of different weight but identical dimensions are made as previously described in Example 1 The first (#3) had an initial weight of 2.50g, and an inital porosity of 75% and was pressed at 3448 kPa (500 psi). The second (#4) had an initial weight of 3.50g and an initial porosity of 66% # and was pressed at 6896 kPa (1000 psi). Both green forms had a thickness of 0.230 cm. Both were sintered at 675°C for 20 minutes in vacuum. # 3 had a final porosity of 66% (a change of -9 % from the inital porosity). # 4 had a final porosity of 54 % (a change of -12%). EXAMPLE 3: Effect of varying the sintering temperature on the density Four green forms were made to the same specification as 1, 2, 3 and 4 as described above, number 5 having the same weight and dimensions as #1, #6 as # 2 and so on. All 4 green forms were sintered at 775 °C for 20 minutes. An additional four green forms (#s 9-12) were made, again as #s 1-4, and sintered in vacuum at 725 °C for 20 minutes. The results of the three examples below clearly indicate that both green form density and sintering temperature have a direct impact on the final porosity of the metal membrane. It is apparent that the more dense the green form, the greater the decrease in porosity due to sintering. It is also quite clear that higher sintering temperatures result in increased shrinkage and lower porosity. Table 1 is a summary of the results. # Temperature(°C) Green Porosity (%) Final Porosity (%) Change from Green 1 675 82 75 -7 2 675 74 63 -11 3 675 75 66 -9 4 675 66 54 -12 5 775 82 68 -14 6 775 74 56 -18 7 775 75 58 -17 8 775 66 42 -24 9 725 82 71 -11 10 725 74 63 -11 11 725 75 63 -12 12 725 66 49 -17 EXAMPLE 4: A preferred fabrication process for making metal powder membranes 70 grams of filamentary nickel powder are sifted into a 12.7 cm diameter mold from a height of 20 cm to give a 0.7 cm-thick bed. This bed is pressed at approximately 2969 kPa [30 kg/cm 2 (430 psi)] to a thickness of 0.4 cm and a density of 1.3 g/cc. The green form has a porosity of 85%. It is sintered at 675 °C for 20 minutes in vacuum. After sintering, the green form (now with a diameter of 11 cm and a thickness of 0.28 cm) is pressed at approx 7117 kPa [72 kg/cm 2 (1032 psi)] to a thickness of 0.25 cm and a density of 2.95 g/cm 3 . The final product has a porosity of 67%.

Claims: A high-porosity metal filter comprising; a membrane element formed by sintering a mass of dendritic metal particles in absence of any extraneous material cohesively binding or supporting said dendritic particles forming said mass; each of said metal particles having intertwined appendages and the intertwined appendages between adjacent sintered dendritic metal particles forming a matrix of interconnected pores; the porosity of said membrane element being defined by said intertwined appendages and being at least 60%. The metal filter of claim 1 wherein said porosity is from about 60% to about 70%. The metal filter of claim 1 wherein said metal particles are nickel. The metal filter of claim 1 further comprising a filter housing having a casing for retaining said membrane element, said casing having an anterior and a posterior section admitting and discharging fluid to be filtered, said element being located between said anterior and posterior sections and being sealably joined to said casing, said housing thereby defining a filtered fluid flow path. A method of making a high-porosity metallic membrane element comprising the steps of; air-laying a substantially uniform low-density bed of a sinterable dendritic metal particles into a mold in the absence of any extraneous material, said mold being suitable for applying compressive force thereto, said bed having a density less than the apparent density of said sinterable dendritic material; compressing said low-density bed to form a green form; and sintering said green form such that after sintering each of the metal particles has intertwined appendages and the intertwined appendages between adjacent metal particles form a matrix of interconnected pores, the porosity of said membrane element being defined by said intertwined appendages and being at least 60%. The method of claim 5 wherein compressing said substantially uniform low-density bed of sinterable dendritic material occurs at a pressure below about 3448 kPa (500 psi). The method of claim 5 wherein compressing said substantially uniform low-density bed occurs at a pressure below about 6896 kPa (1000 psi). The method of claim 5 wherein said sinterable dendritic material comprises nickel The method of claim 5 wherein said metallic membrane element has a porosity from about 60% to about 70%. The method of claim 5 wherein said green form is sintered at a temperature of from about 675° C to about 725°C. The method of claim 5 wherein said membrane element is compressed at a second higher pressure after sintering, thereby imparting additional structural rigidity to said element. The method of claim 5 wherein said membrane element is cut into individual filter elements of a predetermined size. The method of claim 12 wherein said membrane element is cut by wire electric discharge machining.

Inventors: ZELLER ROBERT S, ZELLER, ROBERT S.
Applicants: MILLIPORE CORP, MILLIPORE CORPORATION
Application year: 1994
Main classification: B01D71
IPC codes: B01D39, B01D46, B22F3, B01D71